Groundwater supplies roughly a fourth of the water used in the United States, and roughly two-thirds of the groundwater is used in agriculture. Groundwater reserves vary in type of formation, quality, and location. The law of groundwater allocation and management is primarily left to individual states, and state law and practice vary greatly. After providing an overview of groundwater resources and law frameworks, the presentation will focus on four specific areas of the U.S. as examples: California, Texas, Florida, and the states overlying the High Plains Aquifer in the center of the country. In addition, interstate issues including an interstate groundwater dispute between Tennessee and Mississippi will be discussed. Individual states have adopted one of several groundwater allocation doctrines or combinations of those doctrines, as well as special groundwater management strategies. Texas uses the absolute ownership doctrine, in which landowners own the groundwater underneath the surface, need not have a permit, and can pump without regard to impairment of neighbors. California has adopted the correlative rights approach, which provides a sharing of the groundwater among overlying landowners. Florida uses consumptive use permits in a regulated riparian approach, which authorizes state-issued permits, limited to a prescribed time period and based on the reasonableness of the proposed use of the water. Most of the states overlying the High Plains Aquifer use prior appropriation, which requires permits and holds that first in time, is first in right. In addition to these various doctrines, some states have other laws that provide for prospective management of groundwater withdrawals and attempt to curtail groundwater mining when that situation arises. Kansas enacted legislation in 1972 to enable the formation of groundwater management districts, which aid the state?s chief engineer in setting well-spacing rules and limiting issuance of new permits, when to do so would exceed goals regarding rates of depletion or safe yield. Additional legislation has given states authority through both judges and water administrators to shut down users, partially or entirely, to achieve safe yield in an aquifer. Evaluation on the success of these management techniques varies: supporters think these practices have worked, while critics claim they have been unsuccessful. Cooperation among neighboring states overlying common aquifers has been rare. While interstate compacts covering apportionment of interstate rivers have been common, no compact exists that focuses entirely on groundwater, although a proposal exists for a model interstate groundwater compact. Recent U.S. Supreme Court cases involving interstate river compacts have concluded, however, that even though river compacts fail to mention alluvial groundwater expressly, the compacts cover groundwater, and the Court has ordered upstream states to curtail alluvial groundwater pumping. Recently, the state of Mississippi sued the state of Tennessee in the U.S. Supreme Court, alleging impairment of its groundwater by the city of Memphis, Tennessee. These two states share a common groundwater aquifer, but they have not signed an interstate water allocation compact.